A filter for filtering liquids or gases is generally designed to withstand a certain pressure differential between its upstream and downstream sides. This pressure is usually referred to as the rated pressure of the filter. Over periods of use, dirt and other contaminants in the fluid being filtered accumulate on the surfaces of and within the filter and increase the resistance of flow through the filter, producing an increased pressure differential across the filter. If the accumulation of dirt becomes too great, the pressure differential across the filter may exceed the rated pressure or flow may be impeded. The filter may then become inefficient or rupture or otherwise fail, allowing unfiltered fluid to flow through the filter.
Consequently, a filter system, such as a filter system on an aircraft, can be equipped with a differential pressure transducer for measuring the differential pressure across a filter and generating a corresponding electrical output signal. When the transducer signals an onboard computer that the differential pressure has reached a certain percentage of the rated pressure of the filter, the computer activates a display or other device for alerting an operator of the filter system of the need to clean or replace the filter.
Since the purpose of the differential pressure transducer is to detect when the pressure across the filter reaches a certain value which is lower than the rated pressure of the filter, for reasons of economy and accuracy, the differential pressure transducer frequently is designed to have a lower rated pressure than the filter itself. This design poses no problems if the pressure differential across the filter rises only gradually and steadily as the filter collects dirt. However, during operation of a filtration system, the fluid pressures within the system may fluctuate. For example, on start-up in cold weather, the temperature of the fluid being filtered is lower, its viscosity generally is higher, and the pressure differential across the filter likewise is higher. During these conditions, the pressure differential across the filter may exceed the rated pressure of the differential pressure transducer and damage the transducer.